The IBM intranet (our On Demand Workplace, also known as simply "w3") was recently selected as one of "The Year's 10 Best Intranets" by the Nielsen Norman Group, a leading user experience research company.
Most of us in IBM use w3 everyday for one thing or another. I think the most valuable aspect is our directory - a.k.a. Blue Pages. With such a large company, it's very useful to know who you are talking to (and even what they look like, helps with the conversation). Any company where the workforce is distributed could benefit from that type of directory integration, which pulls info from a variety of sources, although it can be a big project to undertake. Even having a lookup to person's phone number in one place can streamline communication. Blue Pages also allows users to add their own information about customers and projects that they have worked on - and, of course, it's all search-able.
There's a brief case study of the IBM On Demand Workplace here. Of course, WebSphere Portal, Sametime, and Domino figure prominently.
Sr. Product Manager
Workplace Application Development
IBM Intranet Named One of Top Ten